Internal combustion engine valve stems are subjected to bending and reciprocating tensile forces in a direction substantially parallel to the central longitudinal axis of the stem during operation of the engine.
The stress arising from the bending reciprocating tensile forces is generally disposed along a plane in the body of the stem that is perpendicular to the longitudinal axis of the valve stem, and is thus defined as ##EQU1## where the area is the area of the stem perpendicular to the longitudinal axis.
The stem of an engine valve made from metal alloys are apt to warp when exposed to heat treating schedules due to the substantial difference in configuration and mass between the head and stem of the valve. Generally, the warped stems are straightened by application of a transverse force thereagainst while in a heated condition afterwhich the stem's outer surface is ground with a grinding tool whose rotational axis is oriented substantially parallel to the longitudinal axis of the stem which results in circumferential scratch or scribe marks oriented at 90.degree. to the stem's longitudinal axis. Such circumferential scratch or scribe marks promote stress risers that circumscribe and define stress planes that are perpendicular to the stem's longitudinal axis arising from bending and reciprocating tensile forces acting upon the stem during operation of the engine which results in maximum stress since the stress plane area is at its minimum when perpendicular to the stem's longitudinal axis.
Today's increasing use of ceramic engine valves such as those made from silicon nitride pose an even greater problem since their stems are also apt to warp during heat treating but characteristically are unable to be straightened since ceramic characteristically posses a low transverse rupture modulus and would tend to fracture upon application of a transverse straightening force. Accordingly, it has heretofor been the practice to circumferentially grind warped ceramic valve stems to straighten them which again results in stress risers that are generally orthogonal to the stem's longitudinal axis and result in the highest stress arising in the stem due to their having the smallest area as previously described for metal stems.
In addition to the problem of creating stress risers, such circumferential scratch or scribe marks are apt to collect debris such as chafe galling created as a product of frictional engagement between the stem and surrounding guide surface through which the stem reciprocates during operation of the engine.
The inclusion of such debris such as galling chafe in circumferential scratches or scribe marks in the stem's outer surface diminishes the ability to create and maintain an elastohydrodynamic lubrication (EHL) film between the stem and surrounding guide wall surface which can greatly reduce the working life of the valve. As described by Dr. Andrew Jackson in an article entitled "ELASTOHYDRODYNAMIC LUBRICATION (EHL)," beginning on page 833 in "Lubrication Engineering", Oct. 1991; EHL films are fluid films that are very thin, characteristically less than a micrometer in thickness that have high viscosity and prevent surface to surface contact.
In many instances, it has also been common practice to coat the valve stem with a low friction or high wear resistant material such as chrome to lessen friction between the stem and a valve guide through which the stem reciprocates during operation of the engine. Today however, the environmental and toxilogical problems associated with chrome are well recognized and it is to the advantage in the valve industry to lessen or eliminate its use.
The process of present invention addresses both the problems of stress and friction reduction heretofor described which is of particular advantage because of the present trend to higher speed and higher operating temperature engines.
The process of the present invention provides a valve stem finishing process that produces a round and extremely straight valve stem with an advantageous "by-product" of preferentially orienting marks or scratches on the surface of the stem that can maximize EHL film establishment and maintenance while minimizing stress concentration.